Memory is one type of integrated circuitry, and is used in computer systems for storing data. Memory may be fabricated in one or more arrays of individual memory cells. Memory cells may be written to, or read from, using digit lines (which may also be referred to as bit lines, data lines, sense lines, or data/sense lines) and access lines (which may also be referred to as word lines). The digit lines may conductively interconnect memory cells along columns of the array, and the access lines may conductively interconnect memory cells along rows of the array. Each memory cell may be uniquely addressed through the combination of a digit line and an access line.
Memory cells may be volatile or non-volatile. Non-volatile memory cells can store data for extended periods of time, in many instances including when the computer is turned off. Volatile memory dissipates and therefore requires being refreshed/rewritten, in many instances multiple times per second. Regardless, memory cells are configured to retain or store memory in at least two different selectable states. In a binary system, the states are considered as either a “0” or a “1”. In other systems, at least some individual memory cells may be configured to store more than two levels or states of information.
A field effect transistor is one type of electronic component that may be used in a memory cell. These transistors comprise a pair of conductive source/drain regions having a semiconductive channel region there-between. A conductive gate is adjacent the channel region and separated there-from by a thin gate dielectric. Application of a suitable voltage to the gate allows current to flow from one of the source/drain regions to the other through the channel region. When the voltage is removed from the gate, current is largely prevented from flowing through the channel region. Field-effect transistors may also include additional structure, for example reversibly programmable charge storage regions as part of the gate construction. Transistors other than field-effect transistors, for example bipolar transistors, may additionally or alternately be used in memory cells. Transistors may be used in many types of memory. Further, transistors may be used and formed in arrays other than memory.
One type of transistor is a ferroelectric field effect transistor (FeFET), wherein the gate dielectric is ferroelectric. The polarization of the ferroelectric, aligned by applying a programming gate voltage, modifies the conductivity of the semiconductive channel between source and drain for a selected operating gate voltage. A suitable positive programming voltage directs the polarization along the semiconducting channel. This polarization of ferroelectric results in positive sheet charge closer to the channel and negative sheet charge closer to the gate. When considering a p-type semiconductor, accumulation of electrons at the interface occurs to compensate this ferroelectric charge. A low resistivity channel is thereby created. When switching the polarization to its other stable state, the ferroelectric polarization is aligned such that negative sheet charge is closer to the channel and the electrons in the semiconductive channel close the gate dielectric get depleted. This leads to high resistivity. The preference for high and low conductance, invoked by the ferroelectric polarization state, remains after removal of the programming gate voltage (at least for a time). The status of the channel can be read by applying a small drain voltage which does not disturb the ferroelectric polarization.
However, FeFETs can uncontrollably become depolarized, and hence lose a program state. Further, very high electric fields may exist between a typical thin oxide that is between the ferroelectric dielectric material and the channel causing reliability problems in operation.